The present invention relates to an integrated interface circuit comprising an operational amplifier between the output of each output amplifier of the RGB matrix and the input of each video output stage driving the picture tube of a color-television set, and subcircuits for brightness adjustment, blanking, external-signal gating, white-level adjustment, and blanking-voltage adjustment, as set forth in the preamble of claim 1. Such interface circuits are disclosed, for example, in the printed publication "Valvo-Entwicklungsmitteilungen", No. 77, December 1979 ("Vollst/andiger PAL-Decoder mit der integrierten Schaltung TDA 3560"), particularly on pages 8 to 12 and 18 to 20.
In the prior art arrangement, the external signal is gated via a capacitor, so that the DC level of the gated signals must be recovered by clamping it to the black level of the signals provided by the RGB matrix, see FIG. 9 and text on page 11. As shown in FIG. 10 on page 12, the brightness adjustment is performed in the prior art arrangement by means of a potentiometer as usual, but this potentiometer drives a special operational amplifier whose gated output acts on the input of the operational amplifier between the output of the RGB matrix and the input of the output stage. The latter operational amplifier has a stable gain, and its output drives the video output stages via a limiter stage followed by an output stage. As shown in FIG. 15 on page 19, the blanking-voltage and white-level adjustments are effected at the input of the video output stages by means of two trimmer potentiometers one of which is connected in parallel with the base-emitter circuit of the low-end transistor, while the other is inserted between the output of the aforementioned output stage and the input of this transistor.
It is obvious that in such an arrangement, the blanking-voltage adjustment and the white-level adjustment influence each other so that the manufacturer can make the adjustment only iteratively by adjusting the two trimmer potentiometers in turn. In addition, the external-signal gating and the brightness adjustments in the prior art arrangement require an amount of circuitry which may be cumbersome in large-scale integrated circuits.
The invention as characterized in the claims is intended to remedy this drawback; it therefore solves the problem of how to simplify an interface circuit of the above kind in such a way that the requirements placed on the operation of this circuit are met with a smaller amount of circuitry; in addition, blanking-voltage adjustment and white-level adjustment, in particular, are to be possible without interaction.